Problems with 1981 Seca xj750

Yamaha specced the inner two at 10K Ohms on some models; the Seca 750 being one of them.

 

Yeah, there is rumor to that affect, but I have not seen it in the factory or the Haynes service manuals that I have. I will be down to my sons where I keep my Seca on Thursday and I will measure the resistance then. Should have done it yesterday when I was there.

 

Per Len's catalog:

NOTE: all XJ650, XJ700 non-X, XJ750 non-X, XJ900RK, RL, N/FN, and F models, XJ1100, and all XS1100 models originally used 5K resistance value plug caps on all four cylinders. You can use either the OEM or replacement 5K resistive caps and non-resistive plugs, or 0K non-resistive caps along with 5K resistive plugs in order to maintain the original 5K per plug + cap resistance rating.

My 82 750 Seca came with 5K, although the FSM I have does specify 10K for 2 & 3.

 

Yes, in the original parts books there are listings for "optional" 10K caps "with noise suppression" whatever that means, perhaps used with an factory X1 fairing with built in radio, TV, sofa, and dishwasher (those are big fairings). But I've never seen an original bike or even an original set of used coils with the 10K caps on them (except for XJ550's and XJ700-X models, which do use 10K caps originally).

 

It is listed in the Haynes and the FSM, and it makes sense that they were fitted to the touring bikes with factory radio, or to the police bikes.

The extra resistance greatly reduces the RF signal that the coils produce when they fire. Set a portable AM radio next to the bike and you'll hear a pop every time the coils fire. This is the primary reason why cars have resistor plugs.

 

Rooster53 - I have 11.8 volts at the red/white wire on the TCI. I also have 11.8 volts to both coils on either wire when the key is on. I've done resistance checks on the pickup coils - you mentioned some other test - a ground test? How do I do that. (By the way - I think I have mentioned that all 4 spark plugs are firing when I unscrew them and lay them on the engine and hit the starter.)

Is there a way to measure the intensity of the spark?

I swapped the coils (along with the attached plug wires) from the non-working 81 to the working 82 - resulting in no change - 82 works with 81's coils and 81 does not work with 82's coils.

 

Is it orange to blue in color?
Do you hear it go "snap"?
Does it hurt when you touch it?
All are indicators of good spark. A TCI delivers a lower voltage spark over a longer duration than you'd see from a magneto or a modern electronic ignition.


So.......spark on all four, but no fire.......hmmmm........this is starting to look like a circle I once walked in.

 

could this get condensed into another post with "this is what it does" and "this is what i've done already"
the answer is getting lost in all the verbage

 

That's a decent number, but since there was so much talk about a bad ground you can verify this. When you measure the voltage at the red/white wire (in reference to chassis or battery neg), you are measuring voltage drop across the TCI and any voltage drop that occurs in the ground side of the TCI circuit. So red/white wire = TCI voltage drop + drop across TCI connector contacts, TCI ground wire, splice in main harness, connection to frame, and connection from frame to engine, battery neg cable, and battery terminal to battery.

If you place the DMM positive lead on the black wire at the TCI, and the DMM negative lead on the battery neg post, then you will be measuring the ground side of the TCI. Theoretically. you would want this to be zero, but under a few hundred millivolts I would think would be normal. When you measure at the R/W wire with a voltage of 11.8 volts, and if you had a defective ground, then you could have a case where the TCI is dropping 9.8 volts, and the resistive ground is dropping 2 volts producing a low voltage issue with the TCI.

Or, another simpler way would be to place the DMM pos on the R/W wire on the TCI, and the DMM neg lead on the black wire on the TCI. Any excessive voltage drop from either the pos or neg side would show up in this case.

The caveat to both of those scenarios is you need to be drawing some current so if there is a resistive connection either on the pos or neg side it will be apparent. This normally can be done by checking at power up as at least one coil is usually conducting for approximately 2 seconds, however in your case there is some doubt since you never saw a spark with the 2 second delay. So, check the orange and grey coil wires on the TCI at power up and see if one of them goes low (about 1.5v) for 2 seconds or so. If that is happening, then do the above voltage checks in that first two second period. If neither is going low, then you may need to install a brake light bulb from the R/W wire to the black wire on the main harness TCI mating connector and perform the voltage checks. The brake light bulb will simulate the TCI and you can measure the voltage drop directly across it.

That's cheating as you have significantly reduced the load on the starter. Have you checked battery voltage during cranking to be sure it stays above 9.5 volts with the plugs installed? The best way to check spark is with all four plugs in and use a spare plug checking one at a time with the other 3 still connected. Or, as was suggested earlier did you ever swap the batteries between the two bikes? I read back through the post where you said the battery was strong, but a defective battery will turn the engine over quite well but not have enough voltage to power the TCI properly.

 

If I recall correctly, if you have low voltage at the TCI, say 9 volts or so, it isn't going to work. Yamaha's TCI system is power hungry for sure. If you have a battery that is borderline, it will shut down. Have you had the battery load tested? If not, take it to an autoparts store or battery shop and they can load test it for you.

 

Polock suggested that I create a post to summarize "what it does" and "what I've done already" - so I'll give it a shot.

What is does now: The bike has a very hard time starting if it starts at all. The starter cranks and cranks and cranks but the engine doesn't catch and run. Two days ago I was able to get it to run for about 30 seconds by giving is a lot of choke. But it was struggling an did not run well. If I let the choke off or added any throttle it died. Today I have the carburetor off the bike. If I spray a quick shot of starter fluid in each intake port and then crank the starter, it struggles but does not start. Doing the same to my other Seca and it starts up fast and runs at 4000 rpm for a second until the fuel runs out.

What I 've done: Compression check on all 4 cylinders - 145 to 150 psi per cylinder. Leak-down test - 15 to 18 percent on all four. Cleaned the carbs - twice (carbs works great on my other Seca). Tested coils - 2.7 ohms on primary and 22K ohms on Secondary measuring from cap to cap. I measure almost 29K ohms on one of the coils because they have 10K ohm plug caps.Tested the pickup coils for resistance - seem to be in spec. Tested voltage to the TCI - 11.8 volts. Hooked up a timing light and was able to see that the #1 cylinder is sparking (strobing the light) at the correct location on the reluctor on the crankshaft - within the 2 marks just before TDC. Hooked up the timing light to cylinder #2 and saw that #2 spark is strobing 180 degrees opposite TDC. It seems logical to me that if the timing light is strobing, then there must be current flowing from the pickup coils to the TCI to the ignition coils to the plugs and the timing of the engine must be correct.

I have a 1982 Seca that I call the Black Widow because it's black and red. My newest Seca is an 81 and I have named it "Chucky" because it's the cause of my nightmares. I've included a picture of the Widow here. I've swapped many parts from one bike to the other trying to identify the problem component. This morning I swapped the batteries and the Widow ran fine with Chucky's battery but there was no change in Chucky.

Currently the Widow has the following Chucky parts and Chucky has the Widow's parts. The Widow is being driving by my son today and it's running better than it ever has with Chucky's parts. Chucky sits in the garage keeping the 1982 Virago converted to a Cafe Racer company...
1) Both coils and high tension leads
2) Battery
3) Caburetor
4) TCI, regulator/rectifier and the relay that is connected to the tringular panel where the TCI lives

The only other thing I can think of that I can swap over would be the pickup coils. But I've already confirmed the timing is correct based on the timing light.

I have a few more tests I need to do that Rooster53 told me to do. I'll do those tonight.
This is the Widow (should rename it to Frankenstein based on all the new parts she's got now...)

 

One thing That I'm not seeing is mention of the valve clearances having been checked. Have they been?
The Widow looks sweet. You should give us a thread with some more pics of her (or become a premium member and start a showcase)

 

Thanks for the positive comments K-moe. I'm learning a lot but I feel I'm wasting everybody's time 'cuz I'm not finding anything wrong. I did a valve check back on June 9th (buried somewhere in this thread) the intakes were all .005 & .006 and the exhausts were all .007 & .008

 

i am going with valve timing for now. i saw a guy take the cams out of a good running bike to check the shim numbers and when it got together it acted just like this. the exhaust cam was one tooth off.
i know the #'s are good but gauges can lie.
if it was a low battery, once it started it should have made enough voltage to run itself, but maybe not (another problem?)
5K wires, 10K, 0K, it should start, that can be taken care of later.
did you ever put new plugs in it, their cheap.
would you say the plugs are wet or dry after you try starting it?
it really runs like a gazelle and your just messing with us

 

Polock

It's even funnier than that except the joke is on me... not on you guys. I have five bikes - all older model yamahas. Normally when I buy them there is something wrong. For example the Widow cost $250 but it didn't run. Now it runs great. So after fixing four bikes I was ready to treat myself with a good looking bike that ran well and drive it 2400 miles on a summer adventure... now that bike sits... refusing to run. But I'm also an addict... even now I've got my eye on a 1983 Seca 900 about 400 miles from here...

 

....

 

+1 on that.
I know you have said several times that the cam timing is correct and I don't want to be insulting or anything - but are you really really sure?
One thing that caught my attention in one of your earlier posts was that "if you get it started, it dies as soon as you apply any throttle". That doesn't sound like an electrical problem, it sounds like either carbeuration or valve timing. And swapping carbs from one bike to the other is pretty solid proof that it isn't the carbs.
Presumably you've also checked for vacuum leaks in the vacuum line to the petcock, on the carb manifold boots? It would have to be a pretty bad vacuum leak to make a problem like you are experiencing, though.

And the Black Widow looks awesome.

 

I agree on the cam timing. It just acts like it is timed late. A Seca 900? I almost bought one a few years back, but I have too many bikes already.........

 

Agree! I want to see a head-on view to see what you did between the headlight (which looks round, BTW) and the Atari. Also, she looks pretty low! Is that the angle of the pic, or if not, what length shocks on there? Also, did you know there is an Avenger named Black Widow, played by Scarlett Johansen? So yummy, but she's gonna cost more than 250$!

 

Nice bike.

Gary H.